Light alkane aromatization using ZSM-5 based catalysts: Application of density functional theory

Yogesh V Joshi, Purdue University

Abstract

Shape selective zeolite-based catalysts have several applications in the industrial hydrocarbon processing technologies. Propane aromatization is one such process in which light alkanes like propane is converted into value added aromatics (BTX). This dissertation explores the application of the modern molecular simulation methods to correlate the structure of the zeolite to its catalytic performance. These correlations are essential for understanding the role of the catalyst through kinetic models. They are even more important for designing improved catalytic materials using catalyst chemistry models. This dissertation will particularly focus on Ga/H-ZSM-5, which is an efficient catalyst for propane aromatization reaction. Propane aromatization consists of several reaction families such as (1) Dehydrogenation, (2) Cracking, (3) Oligomerization and (4) Cyclization. We have studied the cyclization mechanism using the hybrid molecular modeling (QM/MM) methods. Our investigations starting from C6, C7 and C8 dienes indicated that the cyclization of C7 dienes should be faster than that of C6 dienes. This suggests the possible kinetic role in the experimentally observed higher toluene selectivity. This understanding of cyclization mechanism is essential in order to manipulate the aromatics distribution through search for optimum catalyst. Initial dehydrogenation is extremely important in determining the conversion and selectivity. Using electronic Density Functional Theory methods, we have studied the catalytic activity of extra-framework [GaH]2+ species in the proximity of the two framework Al, and we propose this as a likely model for the most active Ga-related sites for alkane dehydrogenation. We find that the dehydrogenation activation barriers correlate strongly with reducibility of the [GaH]2+ site—and hence the Al-Al distance in the pair-Al site model—consistent with Bronsted-Evans-Polanyi relationships. The optimal Al-Al separation is governed by the interplay between two compensating reaction steps (C-H activation and H-H removal), exemplifying the applicability of the Sabatier principle for a distribution of the Al-pair sites in the zeolite. These structure activity relationships combined with group additivity values will allow us to optimize the catalyst performance by changing Si/Al ratio, Ga loading, Al distribution topologies and zeolite frameworks. In simple words, it provides catalyst chemistry model for inverse search procedure under rational catalyst design framework.

Degree

Ph.D.

Advisors

Thomson, Purdue University.

Subject Area

Chemistry|Chemical engineering

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